This invention relates generally to an improved way to pour molten metal used in a casting operation, and more particularly to minimize the metal damage due to filling of shot sleeve of a horizontal high pressure die casting machine by using bottom filling of the shot sleeve and removal of inclusions present from the dip well.
Low process cost, close dimensional tolerances (near-net-shape) and smooth surface finishes are all desirable attributes that make high pressure die casting (HPDC) a widely used process for the mass production of metal components. By way of example, manufacturers in the automobile industry use HPDC to produce near-net-shape aluminum alloy castings for engine, transmission and structural components. In a typical HPDC process, molten metal is introduced into shaped mold cavities through two metal transfer steps: a (first) low pressure tilt pour from a ladle to a filler tube (called a shot sleeve), and a (second) high pressure injection (such as upon movement of a piston in the tube) into the gating/casting cavity.
The pouring of a molten material, such as metal, for example, into a casting mold is a significant process variable that influences the internal soundness, surface conditions, and mechanical properties, such as tensile strength, porosity, percent elongation and hardness, of a cast object. Many different designs for dipping/pouring ladles exist and are used in the foundry industry. The designs are normally chosen based upon the type of molten metal and casting mold used. Commonly used ladles make use of a slot, a lip and a baffle, or a dam at the top of the ladle to reduce inclusion of furnace metal oxides during metal filling, or the ladle may incorporate a stopper rod to control the flow of metal into and out of the ladle.
Aluminum alloy castings are sensitive to molten metal delivery speed. Molten metals such as aluminum, for example, react with the air and create oxides, commonly known as dross, which upon mixing with the rest of the molten metal creates inclusions and highly porous regions in the cast object during solidification of the metal. When the delivery speed is too low, misruns and cold shuts may result; when it is too high, turbulent flow can entrap air or other gases that can in turn lead to oxide formations, as well as form surface molten aluminum that oxidizes when it comes in contact with ambient air. While many factors influence and account for undesirable properties in the cast object, two common sources of inclusions include formation of a dross layer on top of the molten metal, and the folding action of the molten metal caused by turbulent flow of the molten metal during pouring. Turbulent metal flow exposes the molten metal surface area to the air which creates the dross layer. Depending on the velocity of the molten metal, dictated by the pouring ladle and shot sleeve design and use, the molten metal may fold-over itself many times, thereby trapping oxygen and metal oxide layers therein and exposing additional surface area of the metal to the air.
The concern over higher speed HPDC operations—while more efficient for large-scale production than their low-speed counterparts—is particularly acute considering that the high velocities are an inherent part of the higher delivery pressures. Both the entrapped (i.e., bi-film) and surface (i.e., top-layer) dross mix and subsequently solidify with the rest of the molten metal, which in turn leads to inclusions and highly porous regions that adversely impact structural and mechanical properties of the cast component.
Research has shown that the entrained air (i.e., bi-film) variant of dross can arise if the velocity of the liquid metal is sufficiently high, and that such a velocity is believed to be between 0.45 m/s and 0.5 m/s for Al, Mg, Ti and Fe alloys. See, for example, Campbell, Castings (Elsevier Butterworth-Heinemann, 2003). Thus, it is desirable to keep metal delivery speeds under this critical velocity to significantly reduce the number of oxides being formed in the casting. Maintaining a low metal velocity below the critical velocity is not achievable in a standard tilt pour filling operation of a horizontal shot sleeve because of the required height in which it is poured. The typical free fall velocity of the aluminum alloy stream reaches over 2.5 m/s, five times higher than the recommended velocity. This metal damage is additive to the damage done during the high pressure injection phase.
Typical foundry ladles are referred to as tilt-pour ladles. These ladles are substantially cylindrical in shape with an external spout extending outwardly from the top thereof. The molten metal is typically transferred from the ladle to a casting mold through a pour basin. Turbulence of the molten metal also results when the molten metal is poured through the air and into the pour basin. One method of eliminating this turbulence is described in U.S. Pat. No. 8,522,857 for “Ladle for Molten Metal.” A ladle couples to the mold gating system and rotates to raise the metal above the junction. Two mold pieces are used to form the sprue and coupling orifice. This technology eliminates the need for a pour basin and the free falling metal stream. Its implementation to the filling of a horizontal shot sleeve is deterred by its one piece construction and lack of accessible parting lines.
Porous ceramic foam materials have been used in metal melting furnaces and gravity pour gating systems. Filter efficiency in cleaning molten metal is described in U.S. Pat. No. 3,893,917 for “Molten Metal Filter”, U.S. Pat. No. 3,962,081 for “Ceramic Foam Filter”, and U.S. Pat. No. 4,056,506 for “Method of Preparing Molten Metal Filter”. The addition of filters in low pressure and gravity pour casting molds has been successfully implemented. Mold and core prints allow a filter to be seated in the metal flow path close to the casting cavity, reducing the metal velocity and capturing inclusions. However, there is no feature similar to the mold and core prints which allow a filter to be seated in the metal flow path in a horizontal shot sleeve.
There is a continuing need for a production viable method of transferring molten metal from the ladle to a horizontal die casting shot sleeve which minimizes turbulence in the molten metal and militate against inclusions in a cast component.